1. Field of the Invention
The present invention relates to semiconductor sensors forming a strain detection element on or in a semiconductor substrate, and converting the change in the resistance of said strain detection element caused by elastic deformation to an electrical signal, and relates particularly, for example, to a semiconductor pressure sensor used for automotive fuel injection control and a semiconductor acceleration sensor used in antilock brake systems (ABS) or supplemental restraint systems (SRS) in motor vehicles.
2. Description of the Prior Art
is FIG. 5 is a partial cross section diagram showing the internal structure of a semiconductor acceleration sensor according to the prior art. Referring to FIG. 5, semiconductor acceleration sensor 50 is a hermetically sealed package having a cap 51 and a stem 52 formed from cobalt or another metal, and conductive leads 53 for establishing an electrical connection between the package and other external components. The cap 51 is a box-like member of which one of the sides of the greatest area is open. The cap 51 is welded to the stem 52, which is a large plate-like member, in such a manner that the stem 52 closes the open side of the cap 51. Note that the perimeter of the open end of the cap 51 is flanged to facilitate welding to the stem 52.
Through-holes 54 are provided in the stem 52 in a number matching the number of conductive leads 53. Tubes of a hardened glass are inserted to these holes in the stem 52, and the conductive leads 53 are inserted to the glass tubes. Heat is then applied to melt and fuse the glass tubes to the conductive leads 53, forming a glass seal 55 around each lead and fastening each lead in the corresponding through-hole 54 of the stem 52. When the cap 51 is then welded to the stem 52, the inside of the cap 51 is sealed, and the stem 52 and conductive leads 53 are electrically insulated from each other by the glass seals 55.
One end of the sensor chip 56 of the semiconductor acceleration sensor 50 is fastened to a seating 57, forming the fixed end of a cantilever structure. The sensor chip 56 is, for example, an n-type monocrystalline silicon. The back of the sensor chip 56 is etched to form a thin-wall diaphragm 58, on the surface of which is formed an acceleration detection element 60.
The acceleration detection element 60 is formed by forming four resistances (piezoresistances) utilizing the piezoresistance effect by thermal diffusing or ion injecting boron or another p-type impurity to the surface of the diaphragm 58. The four resistances are then wired together into a bridge circuit by means of aluminum leads formed, for example, by vapor deposition, or diffusion leads formed by doping a high concentration p-type impurity to the diaphragm surface. Stress is thus concentrated on the piezoresistances.
When stress caused by acceleration acts on the sensor chip 56, the sensor chip 56 deflects at the diaphragm 58, thus producing strain in the diaphragm 58. The resistance of the piezoresistances changes according to the rate of acceleration, and an unbalanced voltage is generated at the output terminal of the bridge circuit if a voltage is preapplied to the bridge circuit. The acceleration can then be detected from this unbalanced voltage (the "acceleration signal" below). The acceleration signal, however, is a very small signal, and a signal processing circuit 61 such as a signal amplification circuit, diagnosis circuit, or error detection circuit, is formed at the fixed-end side of the sensor chip 56.
The signal processing circuit 61 is connected by gold or aluminum bonding wires 62 to a hybrid IC 63 comprising a sensitivity adjusting thick-film resistance or offset-adjusting thick-film resistance. The hybrid IC 63 is further connected by bonding wires 62 to the conductive leads 53. As a result, the acceleration signal amplified by the signal processing circuit 61 is first corrected by the hybrid IC 63, and then output from conductive leads 53 to an external microcomputer or other device.
FIG. 6 is a circuit diagram of a conventional amplifier circuit in the semiconductor acceleration sensor 50 described above. Referring to FIG. 6, the acceleration signal output from the acceleration detection element 60 comprising four piezoresistances 65 is differentially amplified by the operational amplifier 70 of the differential amplification stage, temperature-corrected and inversion amplified by the operational amplifier 71 of the temperature correction stage, and is again inversion amplified by the operational amplifier 73 of the final amplification stage before being output. Note that the output terminal of the operational amplifier 71 of the temperature correction stage is connected through resistance 72 to the inverting input terminal of the operational amplifier 73 of the final amplification stage, the output terminal of said operational amplifier 73 is connected through resistance 74 to the inverting input terminal, and the output terminal of said operational amplifier 73 functions as the output terminal of the semiconductor acceleration sensor 50.
FIG. 7 is a circuit diagram showing the output section of the operational amplifier 73. As shown in FIG. 7, the output terminal of the operational amplifier 73 is pulled up to the supply voltage Vcc by resistance 80. Resistance 80 forms a supply path for the current output from the output terminal of the operational amplifier 73, and is used for self-diagnosis of the semiconductor acceleration sensor 50. Note that an equivalent circuit as shown in FIG. 8 is formed when over-acceleration acts on the semiconductor acceleration sensor 50; the voltage input from the operational amplifier 70 of the differential amplification stage to the inverting input terminal of the operational amplifier 71 of the temperature correction stage increases; the output voltage of the operational amplifier 71 of the temperature correction stage drops and the NPN transistor Q81 of the operational amplifier 73 of the final amplification stage becomes on; the voltage output from the output terminal of the operational amplifier 73, i.e., the output voltage Vout of the semiconductor acceleration sensor 50, thus becomes saturated, and the output voltage of the operational amplifier 71 equals the ground level.
The output voltage Vout from the output terminal of the semiconductor acceleration sensor 50 is thus dependent upon the voltage dividing ratio of resistance 72, resistance 74, and resistance 80, and can be calculated using equation 1! EQU Vout=Vcc.times.(R72+R74)/(R72+R74+R80) 1!
where R72, R74, and R80 are the resistance values resistances 72, 74, and 80, respectively.
FIG. 9 is a graph showing the relationship between the output voltage from the operational amplifier 71 of the temperature correction stage, and the output voltage Vout of the semiconductor acceleration sensor 50. When the output voltage from the operational amplifier 71 drops below the voltage level .alpha. at which the output voltage Vout is saturated, what should be saturated by output voltage Vout is not saturated. When the output voltage of the operational amplifier 71 drops further to the ground level, output voltage Vout is as defined by equation 1!. In other words, even though acceleration causing the output voltage Vout to be saturated is acting on the semiconductor acceleration sensor 50, the semiconductor acceleration sensor 50 detects acceleration to be less than the true acceleration.